optimal rpm's on a 91 Johnson 90 hp

[klpaul]

I have a 90hp (V90SLEE)on a 19ft Sunbird. I am new to this and haven't a clue about two strokes in general. I have read that these engines need to achieve 5500 - 6,000 RPM's in order to reduce combustion temps and eliminate coking problems. I really don't want to wind up with a rebuild scenario. Anyone have any advice on what I can do to eliminate this possibility by increasing the RPM's or otherwise? First, what and how to decoke (is that like decarbing?) and next how to achieve the optimal RPM's? klpaul@allphatireandauto.com

 

[klpaul]

Re: optimal rpm's on a 91 Johnson 90 hp

Wrong e-mail so ignore above, my typing skills are about as good as my outboard knowledge.

 

[emdsapmgr]

Re: optimal rpm's on a 91 Johnson 90 hp

Your engine was hp rated at 5000, with an operating range of 4500 to 5500. I would prop the engine run at 5500 with light load, fully trimmed. This engine runs out of breathing capability just over 5500. I would use a can of engine tuner on the engine once each year.

 

[Dhadley]

Re: optimal rpm's on a 91 Johnson 90 hp

We've found that with todays fuel that motor will live a lot longer if it's set up at 5800 with an average load. As always, fresh 87 octane, good oil and Champion plugs are a must.

 

[klpaul]

Re: optimal rpm's on a 91 Johnson 90 hp

Thank you for the help so far. I will get a can of engine tuner this week. My goal now is to get 5800 rpm's over the winter. Right now I am maxing at 4500 WOT and although my speedometer cable is broke (another samll project) there is no way I am breaking 32 and more realistic about 28. Regardless, I have low speed and it is allful out of the whole. The prop will be replaced (17 pitch) but I thought I should focus on the RPM's first to then help determine the optimal prop. Thank you DHADLEY so much, I have new champions, wires, and coil packs arriving later this week (the coil pack insulation has broken down). What would the next step be as I don't think this will get me to 5800 or even close. Carb has been recently rebulit and everything else has been resealed.

 

[wilde1j]

Re: optimal rpm's on a 91 Johnson 90 hp

What pitch prop gave 4500 rpm?<br /><br />Your boat appears to be marginally powered and may need even lower pitch than 17". You'll never get a ton of speed with your boat/motor combination, but you need to do everything to improve the situation before changing props. The things you mentioned are fine and you might consider raising the motor some as well, which may yield some more speed and rpm. If ther motor cavitates easily, it's too high.<br /><br />Let us know where you're at with prop pitch and rpm.

 

[klpaul]

Re: optimal rpm's on a 91 Johnson 90 hp

The current prop is 17 pitch and I want to replace only after I have raised the RPM's unless you advise otherwise. The coils, plugs, wires, and oil change done and will test Friday. I will let you know. I also forgot to note that it has whail tail type stabilzer made of a hard rubber or soft plastic. Should I remove this before Friday as well?

 

[klpaul]

Re: optimal rpm's on a 91 Johnson 90 hp

Here's the deal, just ran the boat after using engine tuner sprayed into the carbs and then the cylineders. Replaced coil packs, wires and spark plugs and chnaged lower case oil. I also removed the whale tail. The result was no change, still 4500 rpm's now just slower to plane. The interesting thing is that when I first started out it ran at about 5100 RPM's for maybe 30 seconds. It then dropped down. I figured it was just due to the cleaning agent left in the cylinders and carb. I am clueless, It doesn't feel like it is misfiring but is this something I can feel like in a car?